Catalytic process for converting terpenes to p-cymene



Patented May 7,,

TERPENES T PM i Washington Hull, Noroton Heights, Conn, as-

' or to American Cyanamid Company, New York,

N. Y., a corporation of Maine No Drawing. 0

riginal application January 9,

1943, Serial No. 471,878. Divided and this application December 11, 1943, Serial No. 513,992

' 4 Claims.-

This invention relates to the catalytic dehydrogenation of terpenes. More particularly, the invention relates to the production of para-cymene by the vapor phase dehydrogenation of such monocyclic terpenes as dipentene limonene;

terpinolene, terpinenes and the like, in the presence of a suitable catalyst containing chromium oxide and oxide of copper and/or zinc. The present invention constitutes a division of the subject matter contained in my copending appli. cation Serial No. 471,878, filed January 9, 1943.

Due to the growing importance of cymen as an intermediate in the production of such sty-' renes as methyl and a-i-dimethyl styrenes, and as an intermediate in the preparations of nitrogenous gasoline additives, increasing interest has been shown in its preparation from terpenes. The possibility of converting a monocyclic ter.- pene such as 'dipentene to paracymene according to the theoretical equation H: Hi

' 2k 2k no on, -m no on mo ha, at his c n o mo \CH: mo cm io il itHu has long been considered as a method of obtaining the latter on a commercial scale. However, heating alone has proved to be inadequate to. carry out the reaction- As a consequence, considerable work has been done in investigating catalytic methods for promoting the reaction.

Because of its adaptability to continuous production on a large scale, catalytic dehydrogenation in the vapor phase has received particular attention. Unfortunately, while the process is desirable from a procedural point of view, its practice has been subject to a number of drawbacks which in the past have prevented its commercial development.

The principal difllculty encountered in utilizing the process has been in the development of a suitable catalyst. The reaction has been carried out with some degree of success making use of such catalysts. as fuller's earth, kieselguhr, activated carbon and the like. The best results with these catalysts have been obtained by contacting the terpene with the catalyst at reaction temperatures of 100-200 C. in the liquid phase and from about'225-350 C. in the vapor phase. However, these catalysts are too inactive and the conversions obtained are too-low to serve as a suitable basis for commercial development.

When attempts were made touse the higher operating temperatures with these catalysts} the final results were poorr While it might be generally supposed that the conversion to cymene could be improved by raising the reaction temperature, actually with most-catalysts the extent to which cracking occurs is found to increase even more rapidly. An examination of the illustrative equation set forth above shows that the reaction is not simple. Not only is it necessary to remove hydrogen from-some of the carbon molecules but it must also be added to others. Hence the reaction has sometimes been referred to as involving both dehydrogenation and hydrogenation.

The reaction is further complicated by the possibility of cracking both the original terpenes and thewholly or partially dehydrogenated reac...

tion products. Therefore, it is not particularly surprising to flnd that raising the temperature with the catalysts proposed by the art actually reduced the cymene production whereastheproduction of such side reaction products as toluene, ethyl toluene, menthane and the like was increased. In some cases considerable lsoprene is found. As a result, it was thought that temperatures above about 325 0. could not be safely used.

acceptable solution to the difllculty, Metallic catalysts, such as nickel or copper for example,

are too active. In order to prevent excessive cracking and tar formation they must be used at such low temperatures in producing para-cymene that the equilibrium conditions are unfavorable. Metallic catalysts of these types as well as the easily reducible metal oxides are particularly troublesome in that they tend to crack of! the isopropyl 8roup,

Many catalysts were found to produce a disproportionate yield of para-menthane. Thi lat ter is particularly unfortunate since once the.

Nor did attempts to make use of more active types of dehydrogenation catalysts provide an Basedon these experiences the art believed that it was necessary to find a catalyst which would not only give a good conversion without a disproportionate yield of para-menthane, but that it must also iinda catalyst which would be operative at temperatures low enough to prevent cracking 'difilculties. One process, developed by research along these lines, is set forth in my U. 8. Patent 2,272,711. In that case it was shown that by the use of a finely divided palladium metal catalyst on such surface active supports as activated carbon and alumina good results can be obtained. Excellent conversion of the terpene to a product remarkably free from such cycloaliphatic compounds as para-menthane were obtained using reaction temperatures of about.

These catalysts, while giving excellent results are subject to a number of inherent drawbacks from the point of view of commercial development. The catalyst itself is, from an industrial point of view, relatively expensive to prepare. Further, since a number of materials'tend to poison the catalyst, it is desirable to feed a material of high purity in order to prevent undue shortening of the active life of the metal catalyst. In addition, they can not be readily activated merely by heating in the presence of air, aprocedure which is easily carried out and is desirableinacommercial process.

Where it is desirable to produce a product of particularly high purity, the palladium metal catalysts are hig y superior. However, from the point of view of commercial production on a large scale, ease of preparation .at low cost and ease of reactivation are such desirable properties in the catalyst to be used that they may often outweigh the added advantage of being able to pro- I duce a more nearly pure product without the necessity for further purification.

Consequently, there remains a demand for a process using a suitable catalyst which will give a good cymene yield, low in para-menthane content, and without an undue amount of cracking the product of the percent of the feed recovered as condensate and the percent converted to cymene. For example, where 75% of the' feed is recovered as condensate containing 80% cymene the index is (75X'80Hl00 Or 60. This enables a comparison of the results obtained when using found to give excellent results. It may be ebbe readily reactivated. It is therefore an object of the present invention to develop such a group of catalyst and a process by which it can be utilized.

In general the objects of the present invention are accomplished by carrying out the dehydrogenation in the vapor phase using a catalyst comprising chromium oxide with or without the addition of one or more auxiliary oxides seiected from the group comprising the oxide of .copper and/or zinc, supported on a non-pyrolytic carrier. Contrary to the teachings of the prior art it was found that it was not necessary to keep the reaction temperature below about'825' was used; This index Leone one-hundredth of tained for example by isomerization of a-pinene, which is in turn obtained from the sulfate turpentine produced as a by-product in the sulfate digestion of coniferous wood for the manufacture of paper pulp. The a-pinene may be isomerized as a separate reaction either in the vapor or liquid phase,.and the vapors of the resultant.

monocyclic terpenes passed directly over the cat alyst of the present invention. -It is not necessary that the monocyclic terpene be pure. 'Accordingly, the products obtained by the isomerization of a-pinene for example may be used directly without fractionation.

Nor is the process of the present invention restricted to the use of a particular apparatus. The materials to be treated must be vaporized and the vaporspassed over the catalyst mass in the reaction chamber at a temperature and rate dependent upon the amountof catalytic mass bein: used. The reaction products may be collected as by condensation and separated into their respective componenets as by fractional distillation. So long as these functions may be performed, the design and exact structure of the apparatus may be varied also at will. The development work was carried out using both glass and stainless steel reaction vessels. However, the

material which is catalytically inactive, does not contaminate the materials and is resistant to intergranular attack by the hydrogen liberated during the reaction may be used.

Two controls were found to be of primary importance in carrying out the reaction successfully. These are the temperature at which the reaction is carried out and the rate at which the vapors are passed over the catalyst. While of the two the temperature is the least critical, both are important in obtaining satisfactory results. It was found that the optimum temperature varies with the nature. of the feed, the amount of feed. the volume of catalyst and the like. Once the approximateoptimum is determined, variations from this optimum of from 25-30 C. in either direction are not particularly objectionable, although the closer the control the better the results. Too high a temperature results in a poor production, due to the lowering of the reaction rate. Too high a temperature results in excessive cracking and ta formation; In general,

temperatures of from about 425-500" C; were found to be satisfactory for a rate of feed within the ranges tested. The invention, however. is not meant to be so limited since in some cases temperatures as high as 525 0., or higherproved tobeuseful.

The rate of feed was found to have the most pr nounced effect on the degree of conversion to the desired products. In general. a feed rate of from about 10 to cc. per hour per 100 cc. of catalyst was found to give good results within the temperature ranges from 425500 0.,- a1- use? though the invention is not necessarily to be solimited; d

All of the catalysts also of a suitable silo and thoxides deposited of the present invention have a particular advantage, in addition to their being relatively inexpensive and easily prepared, in that they can be readily reactivated. Further they exhibit little, if any, decrease in catalytic activity afteras many as thirty cycles. Regeneration is readily carried out by heatins the mass while passing air over it. with some of the catalysts, particularly those containing oxides of tin.

it is well to flush outthe-catalyst with hydrogen after the-burning off. However. this isnot absolutely necessary since acceptable results may be obtained without so doing. V

The method of preparing the catalyst for use in the process of the present invention may be.

varied according to circumstances without departing from the scope of the present invention.

' Preferably it should be done under conditions which promote the lowest apparent density so as to create a large .surface area. In this respectthe catalysts ofthe present invention have an .advantage over those set forth in my copending' application Serial No. 471,877, filed January 9, 1943. In that case the oxide masses themselves were used as the'catalyst. In so doing it is found'that a large proportion of the catalyst mass was not available for the performing of useful worksince it. is substantially impossible to bring the reacting vaporsinto contact with all parts of the surface. The catalysts of the present process, being extended by the use of a nonpyrolytic carrier give a much larger volume of equally effective catalyst for the same weight of oxides since the surface area of the oxides is more emciently utilized.

In addition to the advantage obtained by taking better advantage of the available surface area of the oxides, the catalysts of the present invention have other marked advantages in that thereon. This is probably the easiest and cheapest method and'is therefore preferable. However, in other cases'it is diiilcult to obtain coherent granules of the carrier in sufficient quantity in the proper size range- This dimculty "can be met in any one of several ways. support maybe powdered and made into pellets The upon which the oxide mass is deposited for example, or the oxides' may be mixed with the owder and the whole made into pellets.

Where dimculty in pelleting the material is caused either by the lack of self-adhesion or by the pellets being diflicult to remove from the mold, a binder and/or a. mold lubricant may be found useful. Substantially any combustible, wax-like material may be used for this urpose. Good results may be obtained using a material such as stearic acid in amounts of 1 to 2%. This material is an excellent, mold lubricant and imparts the desired coherence to the particles.

It is easily burned out after the pellets are placed in the converter. Where apermanent binder is necessary, water soluble silicates may be used. Pelleted material is particularly useful in that it enables a uniform packing of the reaction ves- A considerable variation in the nature and propertiesof the oxide which goes to make up the catalyst may also be made without departing from the scope of the present invention.

The chromium oxide which isused alone or in combination with other oxides may be in either the three or six valent form. when used alone the trivalent form appears to give the better results. In admixture, either the three or the six valent form may give the better results depending principally on the makeup of the other oxides present and operating conditions. In many cases the presence of'chromium, as an oxide,in

, both forms is helpful; Ingeneral the chromium they are generally easier to prepare.- Further the materials used as supports are physically stronger than the oxide masses themselves so that the resultant mass is easier to handle both in the process of manufacturing the catalyst and in the conversion processes themselves. The

supported catalysts exhibit less tendency to settie in the reaction chamber and thereby interfere with the free flow of vapors. They'can withstand higher temperatures without physical damage than can the unsupported oxides, so. that quicker and more eiilcient reactivation can be carried out by burning in air.

In preparing thezcatalyst so as to obtain the desired distribution of the oxides on theqcarrier and maintain the lowest bulk density any desirable procedure may be used. For example, a solution of the metal or metals in water may be used to impregnate the carrier. mass after which it can be dried, and the insoluble metal compound reduced; to the corresponding oxide in situ. The carrier inay be impregnated either by spraying on the carrier mass or by adding the carrier mass tothe solution if the latter is suflicient in volume. Another procedure which can be used to obtain satisfactory results is to impregnate the carrier by using a suspension of the metals in their oxide form.- This procedure is ordinarily used where it is diflicult or the facilities are lacking to convert the metal com-. pound to the oxide in place.

The physical form in which the finished catalyst occurs may also vary quite considerably.

In some cases a support may be made into granoxide should be present in from about 1-3 mol parts for from 1-3 mol parts of the oxide or oxides of copper and zinc The auxiliary oxides used in conjunction with the oxides of chromium may be varied considerably, both' in composition and in amount. In developing the process of the present invention it was found that with the optimum conditions of feed rate and temperature, best results were obtained using from 1-3 mol parts of the oxides of copper and zinc per' part of chromium oxide.

The invention, however, is not necessarily so limited. Useful results are obtained using proportions ranging from 1-3 moi parts of.cnromium oxide to about 1-3 parts of the auxiliary metal oxide.

Replacing all or part of the auxiliary metal oxide with an alkaline earth metal oxide such as barium or calcium usually has a slightly beneficial action on the production rate but decreases the yield of cymehe by increasing the cracking rate. The presence of uncombined alkali metal oxides appears to be undesirable in that it pre-- trolled by means of a diluent. This is highly undeslrable since it complicates both the process and apparatus and adds to the cost of operation. In most cases the catalysts of the presa' diluent than they do when such a gas is used in-making" up the catalysts of the present invention. A number of these are the same materials which were used as catalysts in the earlier prior art. Notable among these are kieselguhr and aluminum oxide. The activated carbon catalyst used in the prior art is also a good material except that it cannot be readilyreactivated by burning. Among the more useful supports found were alumina and silica gels, such as the ordinary activated alumina, of commerce and broken particles of silica bricks. Closely analogous to the latter, and also as good in use, are broken particles of burnt clay. The days, however, should be as free as possible-from alkaline metal oxides since these tend to promote excessive dehydrogenatlon and/or cracking. Another useful support is titanium dioxide although the latter suffersfrom the drawback that it must be powderedent invention give better results in the absence I Anumber of non-pyrolytic materials are useful and pelleted to uniform size in order to serve as support for the other oxides.

Of the groups of materialstried and found userul, Activated Alumina is perhaps the most desirable since it is commercially available in forms ready for use, is relatively cheap and exhibits the least tendency among the supports tried to promote cracking. The invention, however, is not meant to be so limited since as pointed out good results were obtained using silica gel granules, broken bumtclay particles, titanium dioxide and kieselguhr as supports.

The invention will be more fully explained in connection with the following examples which are meant to be illustrative only and not by way of limitation.

In the following examples, the reaction products were analyzed to find the degree of conversion by passing dry HCl gas through 25 cc. samples of the condensate in an ice bath at the rate of about 2 bubbles per second for 1 hours. The ice bath was then removed and the gas bubbled through at the same rate for 1 hourat room temperature. Free 1101 was removed by evacuating at 10 mm. of mercury for ,5 hour at room temperature. Approximately 2 gms. of the sample was then treated with 50 cc. of neutral methanol after which 50 cc. of standardized alcoholic. KOH was added and the mixture refluxed for 2 hours. Since the unconverted terpenes form stable hydrochlorldes whereas the desirable products do not, titration of the excess KOH after saponiilcation gives a measure of the terpenes remaining in the condensate.

Exams: 1

.A solution of 52.6 gms..of CrO: in 500 cc. of water to which was added 10 cc. of concentrated sulfuric acid was prepared. 80: gas was bubbled through the solution at the rate of about 2 bubbles per minuteuntil the chromium was reduced to the trivalent form. The solution was heated gently refuse while-the mlss was heated gently. 120 cc. of the resultant product waspacked in an upright column and vaporized dipentene passed thereover at diflerent temperatures and feed rates. Reaction products here collected by condensation and analyzed. Illustrative results are shown in Table I.

Table '1 Temp. av.) C. my Index 25 gm. moles of powdered commercial and 12.5 gm. 'moles of commercial sodium bichromat (NazCraOr-2Hz0) were dissolved in 30 liters of water and approximately 50 gm. moles of ammonia in the form of commercial aqueous ammonia were slowly added. The exact end point was reached when the addition of slow drops of ammonia solution to a filter sample of the solution gave no precipitate. The red-brown precipitate of Cu(0H)NH4CrO4 wa collected and washed on a vacuum filter. The wet paste was then thinnedto the consistency of a heavy lubricating oil by the addition of a small volume of distilled water and mixed with enough 4 to 8 mesh Activated Alumina to absorb all of the paste. This mixture was dried overnight at 125 C. and the resultant granules heated in small portions over a free flame. The application of heat was adjusted to maintaintemperatures of about 300 during the reduction step. when reduction was 340 C. The resulting black granules consisted of a coating of CrzOa in admixture with copper oxide in the ratio CraOa:2CuO coated upon and impregnated into the Activated'Alumina carrier. 100 cc. of these. granules were used in the dehydrogenation of dipentene as in the preceding example. Illustrative results are shown in Table II.

Table II AV. Temp. (av.) C. com," Index percent 20 as 20 89 8O 26 86 7B 25 84 76 40 83 76 50 83 77 Exam 3 4 gm. mols of ZnO was dissolved in 50 percent nitric acid. The solution was stirred and sumcient aqueous ammonia to render the solution basic was gradually added while keeping the temperature below about 18 C.- The resultant precipitate was collected, washed free of nitric acid, and suspended in about 80 mols of water to which 2 mols of CrO: in an additional 30 mols of water was added. The resultant solution was agitated at about 40-50" C. for approximately two hours and then evaporated at 80-90 C. to a thin paste. This paste was then added to about 62 gm. mols of 6 to 12 mesh activated alumina, addition being made in three successive parts with complete dryin: between each application. The-mass was then finally heated to constant welsht at about i an mthosmnredwas elehydrocensting dipentene as in the Table III Tanp. (an) 0; Feed, Anson, I I

. 4 s 7: I w

an n m Q 91 70 49 1: cs 4. U 18 ,70

at from about 4Q0-525 0,, completely vaporisf in: s. monocyclic terpene hsving glnglg propyl side chain at a rate of from 0.1-to1.0 parts of liquid terpeneper port oi catalyst by volume,

assing the completely vaporized terpene over the heated catalyst, whereby a major portion of the usedpeeedint Typiealresultsare shown in'hhle in.

terpene is converted to o-cymcne, condensing the condensable portion of the reacted vapors and isolating the p-eymene content thereof; said cataiystbodyconsistinzoifmm 1-.-3 molllartsotone member, selected from the soup eomis'ting of s cnoi grot and-mixturesofthe sameJwether poitheozidcsofoopneranclzinc, heingsupportedonncerrierhsving a CrzOa, 0:0: and mixtures of the same, together.

with 1-3 moi parts or copper oxide, said oxides beinc supported on a carrier having a high surface activity but exhibiting substantially no tendenc'y to remove the isopropyl side chain from the terpene at the operating temperature.

.4; A process according to claim 1 in which the carrier is "Activated Alumina.

WASHINGTON HULL.

moipartsotanotideselectedfromthe surface activity hut exhihitins suhstaniinlly V Patent No. 2,399,739.

It is hereby certified that error appe numbered patent requirin Too high read Too low; a

correction therein that the sam Oflice.

Signed and sealed this g correction as Certificate of Correction WASHINGTON HULL are in the printed specification 0 follows: Page 2, seco f the above nd column, nd that the said Letters Patent line 60, for should be read with this e may conform to the record of the case in the Patent 16th day of September, A. D. 1947.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

